The present invention falls into a class of technology and methods where gasborne charge-carriers are used to neutralize a charge imbalance on insulating materials and floating conductors. The methods are applied in general industry for static elimination to reduce hazardous and nuisance static discharges and improve process operations and cleanliness.
Electrical static eliminators are used in many industries to control unbalanced charges on insulating materials and floating conductors. FIG. 1 shows one example of a prior art static eliminator system including positive and negative polarity corona ionizers 1, their environment 10, and a target 11. When the ionizers 1 are distant from the target 11, gas flow 7 is used to convey the products of ionization to the target. The corona ionizers 1 can be separate dc or pulsed-dc emitters, or single emitters with alternating potential to separate the positive and negative polarity corona in time.
The make-up of ions from a typical ionizer is very complex and is far from understood. Many species are short-lived, and often highly reactive. Most ionic species discussed in the literature are found in the interelectrode gap, after ion molecule reactions have had time to develop. The ions and their distribution also depend on the corona mode (e.g. glow or pulsed) that is active for the electrode geometry, the gas, and the potential.
The carriers entrained from a corona by gas flow are only beginning to be explored. However, it is becoming clear that only about 0.1% of carriers generated in a corona are entrained, and the control of these carriers is not achieved by trivial adjustment of positive and negative corona currents.
Conventional charge eliminators produce gasborne charge-carriers of positive and negative polarity, so that the charge needed for static elimination is attracted from the gas to charged articles. The equipment includes nozzles, blowers, and room ionization systems where charged carriers are conveyed from electrical corona to articles to be neutralized. Other ionizers are simply placed in chambers where gas circulation conveys the charge-carriers to electrostatically charged articles, or are static bars fitted with air knives or tubes perforated with an array of orifices. The corona ionizers can consist of separate positive or negative polarity charge-carrier generators for direct current (continuous or pulsed) ionization. Alternatively, the ionizers can be single emitters or arrays of these emitters operated at alternating polarity.
A noted deficiency with conventional ionizers is that they do not perform well in nitrogen, hydrogen, and noble (inert) gases, because control is difficult where the gases are non-electron attaching. These ionizers also use corona electrodes with two separate polarities or alternating polarity.
Nitrogen is used to inert processes in many industries, and can purge areas cooled by the evaporation of liquid nitrogen. In recent years, static eliminators using nuclear (radioisotope), ultraviolet, soft x-ray, and corona discharge ionizers have been explored for use in nitrogen environments. Nitrogen, hydrogen, and the noble gases pose special problems for electrical static eliminators, since the negative carriers formed in the negative corona discharge are free electrons and these do not readily attach to atomic or molecular nitrogen species. In industrial applications, where the impurity is not always well controlled, there will be some electron attachment, and the effective negative-carrier mobilities and negative polarity corona current can vary over great ranges without significant effect and control on carrier entrainment. The mobility effect is also influenced by temperature.
In International PCT Publication No. WO 01/09999 entitled xe2x80x9cIONIZER FOR STATIC ELIMINATION IN VARIABLE ION MOBILITY ENVIRONMENTS,xe2x80x9d designating the United States, now U.S. application Ser. No. 09/762,521, which is incorporated by reference herein, balanced static elimination is achieved in variable ion mobility environments using positive and negative polarity corona emitters. The balance, however, is more difficult to control in high purity nitrogen and at low temperatures where positive carrier generation must occur at higher electric fields where the ratio of negative to positive polarity emitter currents can exceed 1000 to 1.
Each of the alternative technologies (nuclear, UV, x-ray) produces positive ion and free electron pairs in nitrogen. The balance of these ionizers, however, is not easily controlled in air, let alone nitrogen gas and over the temperature range of interest (i.e. 200 degrees K to 450 degrees K). Also, the alternative ionizers can introduce radiation hazards to the work place. X-ray, radioactive and UV ionizers pose radiation hazards in the environment and typically need to be licensed or shielded for use in commercial applications. The corona type electrical ionizer, on the other hand, does not need to be licensed as a source of ionizing radiation, and operates in the current-limited mode throughout its useful life. The performance of the corona type electrical ionizer does not decay over time as will occur for at least the radioactive ionizer. The electrical ionizer is, therefore, preferred if its balance can be controlled.
Many static eliminators have been proposed for use in industrial environments. Some have claimed to be useful in nitrogen environments. U.S. Pat. No. 5,883,934 (Umeda) describes that imbalance in the entrained carriers from ionizers can be based on UV ionizer radiation brought into balance by a dc bias. The same is true for ionizers based on corona ionizer activity and other forms of ionizing radiation, such as UV and radioactive ionizers, which produce carrier pairs. Umeda, however, does not recognize the importance of carrier mobility in bringing about balance in gases such as nitrogen at low temperature. Thus, it is unlikely that balance of this ionizer can be controlled in a non-electron-attaching environment by the method proposed in the patent.
When positive and negative polarity corona emitters are used as the corona source, balance can be achieved by adjusting the potentials on the emitters. The ratio of currents from these emitters is shown in prior art FIG. 6 for gases 213 degrees K and 300 degrees K. The difficulty with the arrangement of prior art ionizers such as those discussed in WO 01/09999, is that the control point (residual potential=0) is achieved at large current ratios or is not achieved at all at lowest temperatures. The ratio of currents needed to achieve balance in nitrogen is shown in prior art FIG. 7 as a function of temperature. The method described in WO 01/09999 achieves the balance by operating the negative emitter at a high current (limited) condition and adding positive-polarity corona current as needed to balance the ionizer.
The present invention departs from conventional technology by relying upon a single polarity corona to generate simultaneously both positive and negative carriers and to balance this ionization using a corona-free dc bias electrode to remove unwanted carriers. The invention is best practiced for use with a negative polarity corona. Negative polarity corona generally contains an extended corona structure that improves contact between positive and negative ions and gas flow, and is especially suited for use in nitrogen, hydrogen, and inert gas environments where there is an intense current-limited discharge. The choice of corona electrode polarity is driven by the higher mobility of the negative carriers and their relative abundance in the corona source.
Many balancing and self-balancing circuits have been developed for electrical ionizers in air, but few have been designed for use in variable ion mobility environments. The present invention offers improvement over existing balancing circuits in nitrogen environments, such as described in International PCT Publication No. WO 00/38484 entitled xe2x80x9cGAS-PURGED IONIZERS AND METHODS OF ACHIEVING STATIC NEUTRALIZATION THEREOF.xe2x80x9d Unlike conventional balancing circuits based on two polarity corona systems, a single-polarity (negative) corona is controlled using a passive (corona-free) control element. The complicated interaction of two corona systems, which could separately have changing corona modes (morphology) is thereby avoided.